In digital transmission systems it is necessary to synchronize a receiver with the incoming digital signal. In carrier systems, information is transmitted in frames and synchronization is realized by properly locating the frames by a process called framing.
In early channel banks framing is achieved by a so-called backward acting reframing arrangement. To this end, the received signal is compared to a locally generated framing pattern. When the comparison indicates errors are occurring, a search is initiated in order to reframe. In D1 systems the search includes slipping one bit at a time in order to locate the true framing pattern. In D2 and D3 systems, groups of eight bits are checked at a time in order to speed up the reframing process.
More recently, a system has been proposed as disclosed in U.S. Pat. No. 4,125,745 issued Nov. 14, 1978 in which the even framing bit positions of the presently received and last three DS1 super frames are monitored to detect whether or not a valid combination of the framing bits is present. Upon detection of an invalid combination or combinations which subsist for a prescribed number of errors, the circuit is switched into a search mode wherein all the bits in the DS1 super frame (i.e., 386 bits) are monitored in an attempt at finding the true framing pattern. Upon detection of a "true" framing pattern the circuit switches back to the monitoring mode.
A problem with the prior arrangements is the possibility of framing on a false framing pattern which emulates the true framing pattern and which is either generated for example, by a subscriber transmitting a data pattern or one which may statistically occur in normal digital transmission. That is to say, a frame resynchronization circuit must be capable of determining whether the receiver is synchronized with a false framing bit pattern.